General notes for all sensors

Always observe local installation and safety regulations.

General notes for all sensors

General notes for all sensors

Do not install sensors so that they protrude in any direction, and do not suspend them below the horizontal. Ensure that they are protected against damage and vandalism and will not cause injury.

Be aware of the effects of orientation on the functioning of the sensor.

Always determine the following before mounting: ■ Min./max. ambient temperature ■ Ambient humidity and exposure to spray water ■ Exposure to vibration ■ Explosion protection ■ External influences 1

General notes for all sensors

Take account of the active and inactive sections of a sensor probe.

A tight-sealing test-hole must be provided adjacent to every sensor.

The cable should be installed with a "drip loop" to prevent water from entering the sensor housing.

2

General notes for all sensors

Always allow a sufficient length of spare cable so that the sensor can be removed at any time without disconnecting the wiring.

When installing a sensor, avoid compressing the lagging.

Use a graduated-diameter mounting flange to avoid compressing the lagging.

If the mounting flange supplied does not have the appropriate graduations, use spacing bushes to avoid compressing the lagging.

3

General notes for all sensors

If the sensors are to be concealed (e.g. in false ceilings, shafts etc.), mark their locations visibly and record them in the site documentation.

Fix a labelling plate in the direct vicinity of the sensor. This must include a plain text description and the reference number, which appears in the plant schematic. Do not attach the label to the device itself.

4

Immersion sensors · Water

Temperature · Immersion sensors

Temperature

Ensure that the full active length of the sensor is immersed in the medium.

Incorrect installation

5

Temperature · Immersion sensors

Plastic sleeve

Chilled water and refrigeration pipes To prevent condensation, extend the immersion pocket inside the lagging by use of a plastic sleeve.

6

Temperature · Immersion sensors Water-resistant seal

Chilled water and refrigeration pipes The hole in the lagging must be sealed, to prevent the ingress of moisture (water-resistant seal).

7

Temperature · Immersion sensors

Install sensors against the direction of flow.

Incorrect installation

Install at the correct angle.

8

Temperature · Immersion sensors

If the active length (a) of the sensor probe is longer than the diameter of the pipe, the sensor should be installed diagonally, or in a bypass pipe or bend.

The inlet side of the bypass pipe should project into the main pipe.

9

Temperature · Immersion sensors

Maintain a clearance (distance A) between the sensor and any obstruction, so that there is room to remove the sensor from the immersion pocket.

For each sensing point, an additional immersion pocket, adjacent to the sensor, must be provided for test purposes.

Install outlet-temperature sensors directly at the heat exchanger outlet.

10

Label

Installed without immersion pocket

Temperature · Immersion sensors

Sensors mounted without immersion pockets or with slotted or perforated immersion pockets must be identified accordingly. Attach a label marked: Installed without immersion pocket.

Distance from mixing point to sensor minimum maximum

10 x d 15 x d

Distance from mixing point to sensor minimum maximum

10 x d 15 x d

When mixing water at different temperatures, always maintain an adequate distance between the mixing point and the sensor (to take account of stratification).

11

Temperature · Cable sensors

Temperature

Cable sensors · Water

The sensor element is not affected by orientation, but must be fully immersed in the medium to be measured (air or water).

Use a file to ensure a smooth, clean contact surface, and fill the space between the sensor and the pipe with heat-conductive compound to improve thermal conductivity.

12

Probe sensors · Air

Temperature · Probe sensors

Temperature

Ensure that the full length of the sensor probe is exposed to the air flow.

A test hole must be provided adjacent to every sensor.

Do not use probe-type sensors in areas where stratification can occur (e.g. downstream of mixing dampers, heating coils, cooling coils or heat recovery units). Averaging sensors should be considered.

13

Temperature · Capillary sensors

Temperature

Capillary sensors with probes

Install so that the device head is higher than the sensor probe.

The sensor probe should be tilted downwards.

g➞ on r w rig ht ➞

14

Temperature · Capillary sensors

The ambient temperature at the device head must always be higher than the temperature to which the sensor probe is exposed.

The sensor element must always point downwards. Do not allow the capillary to form a U-shape.

Do not bend the capillary too tightly (radius of bend must not be less than 50 mm).

When routing the capillary through internal or external walls, for example, always use a lined and insulated conduit.

15

Temperature · Capillary sensors

Any unused length of capillary should be neatly rolled.

Where the capillary passes through sheet metal, protect it with a rubber grommet (to prevent shearing).

16

Averaging sensors

Temperature · Averaging sensors

Temperature

Allow a distance of at least 50 mm between the heat exchanger and the sensor.

The entire length of an averaging sensor must be installed fully inside the air duct.

The sensor element must be distributed evenly over the full cross-section.

17

Temperature · Averaging sensors

If air washers are used for humidification, install the sensor element downstream of the eliminator plate, in the direction of air flow.

18

Temperature · Averaging sensors

Do not bend the capillary too tightly (radius of bend must not be less than 50 mm).

When routing the capillary through internal or external walls, for example, always use a lined and insulated conduit.

Where the capillary passes through sheet metal, protect it with a rubber grommet (to prevent shearing).

Install the sensor element using capillary supports.

19

Temperature · Frost protection

Temperature

Frost protection · Air

Leave a spare capillary loop of 20 cm so that functioning can be tested outside the unit.

If the supply ductwork is outdoors or in an unheated space, then both the measuring head and the test loop of the thermostat must be located inside the duct and downstream of the heat exchanger.

Install the capillary in the direction of air flow, downstream of the first water-filled heating coil exposed to frost. The capillary must be installed diagonal to the heat-exchanger pipes.

20

Temperature · Frost protection

When installing on "drawer"-type units, ensure that the electrical connections are long enough to enable the units to be pulled out.

If a water-filled cooling coil is installed upstream of the first heat exchanger, then the frost protection thermostat must be installed upstream of the cooling coil, in the direction of the air flow.

With large heat exchangers, or with heat exchangers comprising several units, more than one frost thermostat must be installed (minimum 1 per unit).

21

Temperature · Frost protection

Do not bend the capillary too tightly (radius of bend must not be less than 50 mm).

When routing the capillary through internal or external walls, for example, always use a lined and insulated conduit.

Where the capillary passes through sheet metal, protect it with a rubber grommet (to prevent shearing).

Any unused length of capillary should be neatly rolled.

22

Temperature · Frost protection

Use spacing clips to maintain a 50 mm clearance.

Install the sensor element using capillary supports.

23

Temperature · Room

Temperature

Room sensors

Install sensors at a height of 1.5 m in occupied spaces, and at least 50 cm from the adjacent wall.

Do not install where sensor will be exposed to direct solar radiation.

Always use a thermally-insulated backing when fitting to solid walls (steel, concrete etc.)

24

Temperature · Room

Do not install on external walls.

Avoid recesses (e.g. shelving) and alcoves.

Do not install near lamps or above radiators.

Avoid chimney walls.

25

Temperature · Room

Do not install directly adjacent to doors.

Do not install behind curtains.

Do not fit to walls concealing hot-water pipes.

Seal gaps between cable/plastic tubing and conduit. Otherwise measurements will be falsified by incorrect circulation of the air.

26

Outdoor sensors

Temperature · Outdoor

Temperature

The system design determines the façades (N,S,E,W) on which the sensor should be located.

Do not expose to direct solar radiation.

Do not install on façades affected by significant rising heat. Do not install on façades warmed by solar radiation.

27

Temperature · Outdoor

Avoid chimney walls.

Do not install under eaves.

Do not install above windows.

Do not install above ventilation shafts.

28

Otherwise measurements will be falsified by incorrect circulation of the air.

Temperature · Outdoor

Seal gaps between cable/plastic tubing and conduit.

Do not paint the sensor.

Ensure accessibility (for inspection/testing)

29

Temperature · Outdoor

Temperature

Outdoor sensors · Cable sensors

The same rules apply to outdoor cable sensors as to any other outdoor sensors. The cable should be connected from below (to protect it from dripping water).

30

Surface-mounted strap-on sensors

Temperature · Surface

Temperature

The surface must be clean and smooth (remove paint). The sensor must be fixed firmly to the surface. Use heat-conductive compound. Important: Avoid exposure to external heat gains.

Consider cable length when fitting to windows which can be opened! The sensor must be fixed directly to the window surface.

The sensor must be fixed directly to the surface. Use heat-conductive compound.

31

Temperature · Surface

Distance from mixing point to sensor Minimum 10 x d Maximum 15 x d

Distance from mixing point to sensor Minimum 10 x d Maximum 15 x d

When mixing water at different temperatures, always maintain an adequate distance between the mixing point and the sensor (to take account of stratification).

32

Wind sensors

Temperature · Wind

Temperature

Install on the façades exposed to the main wind direction. Make sure the sensor is accessible (for inspection/testing).

Do not install under eaves. Do not install in recesses.

33

Temperature · Solar

Temperature

Solar sensors

Install solar sensors on the façades behind which the associated control system is operative.

Install the sensor where it is easily accessible (for inspection/testing).

34

Temperature · Solar

Avoid shade (from trees or neighbouring houses etc.)

35

Humidity · Duct

Humidity

Duct sensors

Note that humidity sensors are affected by air velocity. The air velocity in the vicinity of the sensor must not exceed 10 m/sec. Precaution: Fit the sensor with a perforated shield or cover (e.g. perforated steel)

Avoid dead-legs. (Supersaturation can occur in areas where there is no air flow.)

Important: When installing sensors in ducts with negative pressure, it is possible for air from an external source to be drawn into the device and the installation hole. (Seal tightly to prevent false readings.)

36

Humidity · Duct

A test hole must be provided for every humidity sensor (downstream of the sensor). Recommended diameter: 40 mm.

For maintenance purposes, the electrical connections should be of the plug-in type (e.g. TT45…).

37

Humidity · Duct

Distance for humidification measurements for BM BM is the distance between the humidifier and the humidity sensor necessary to allow the air to absorb 100% of the water supplied. The required distance depends on the amount of water supplied, the velocity of the air and the type of humidifier system. If the humidity sensor is not mounted at the required distance, it will produce a false reading.

Example: Because it is in the wrong position, the sensor here measures only 30% of the water or steam introduced into the system, as only this amount has been fully absorbed in gaseous form into the air. The sensor element will get wet, produce an incorrect reading, and may be damaged.

38

Humidification systems: Air washers BM downstream of eliminator plate Tray-type humidifier BM 3.5 m Spray rehumidifiers BM 5.5 m Spray humidifiers See water volume diagram (adiabatic) page 39 Ultrasound humidifiers Centrifugal humidifiers Atomizer humidifiers see diagram for steam page 40 Pressurized steam BM = Isotherm “Pressure-free” steam BM = Isotherm · 1.3

Humidity · Duct

Distance for adiabatic humidification measurements This diagram is designed for use in winter, with an absolute humidity content of 1.5 g/kg on the intake side, and a supply air temperature of 18 °C.

Water volume Δ x (g/kg)

Air velocity in the duct or device w (m/s)

Water chart (adiabatic)

Distance for humidification measurements BM (m) Method: Enter the air velocity (in m/s) on the left edge of the diagram (e.g. 2.0 m/s). From this point, draw a line to the right, along the line indicating the increase in humidity (example: 䉭 x = 10 g/kg). Starting where the two lines intersect, draw a vertical line and read the required distance BM for humidification measurements on the horizontal line at the bottom of the diagram (6.7 m). 39

Humidity · Duct

Distance BM for humidification measurements with steam humidifiers A certain distance is required between humidifier and sensor, so that the air has time to absorb the water (vapour) supplied by the humidifier before the sensor measures the humidity. This distance is marked on the diagram as BM. The minimum distance between the humidifier and the humidity sensor must be equivalent to at least BM.

Determining BM Distance for humidification measurement BM (m) = 8.5

Increase in air humidity

䉭 x [g/kg]

Air velocity in duct or device w (m/s)

Method: Enter the increase in humidity in g water/kg air (e.g. 4.5 g/kg) on the right edge of the diagram. Draw a horizontal line extending from this point towards the left. Enter the minimum duct air velocity (in m/s) on the bottom edge of the diagram (e.g. 1.9 m/s) and draw a vertical line extending upwards from this point. From the point of intersection of these two lines, draw a diagonal line extending upwards and parallel to the existing diagonals. Read the distance, BM, in metres, from the scale on the edge of the diagram (example 8.5 m). 40

Humidity · Duct

Humidity sensor

Average humidity measurement Locating the humidity sensor in a bypass duct improves the measurement of average, relative or absolute humidity, and should be used: In cases of temperature or humidity stratification. Here too, the appropriate distance for humidity measurements, BM, must be maintained.

41

Humidity · Room

Humidity

Room sensors

H% Install sensors at a height of 1.5 m in occupied spaces, and at least 50 cm from the adjacent wall.

Do not install where sensor will be exposed to direct sunlight.

Do not install on external walls.

42

Otherwise measurements will be falsified by incorrect circulation of the air.

Humidity · Room

Seal gaps between cable/plastic tubing and conduit.

Do not install near lamps or above radiators.

Avoid chimney walls.

Do not install directly adjacent to doors.

43

Pressure · General

Pressure

General

Pressure sensors are affected by orientation (see manufacturer's installation instructions).

Pressure tubes must be provided with an isolatable T-fitting near the device head for test purposes.

To prevent overload on one side when making adjustments, the connection must always be fitted with an isolating bypass.

44

Pressure · General

Where there is a risk of condensation, the differential pressure tube must be installed at a gradient of 1:30 and fitted with a drain mechanism. The drainage point must be lower than the device head and sensing point. Protect from frost and avoid U-shapes.

Pressure tubes containing circulating air must not be introduced into the open air or routed through cold rooms or ducts. This prevents the risk of condensate freezing in the tubes (e.g. with pneumatic venting sensors).

Mount on a vibration-free surface.

The pressure-tapping point must not be located in turbulent air. Ensure sufficiently long settlingzones upstream and downstream of the tapping point. A settling-zone consists of a straight section of pipe or duct, with no obstructions.

Formula: dg = Equivalent diameter

45

Pressure · Air

Pressure

Air The measuring tip is screwed or glued to the duct wall. Seal to protect from external air. Remove any swarf from the inside of the duct.

Important Protruding fixing screws will impair correct measurement.

Correct installation.

46

Pressure · Air

Avoid using tips which protrude into the duct for static pressure measurements.

Internal diameter mm

Probes are used to measure static pressure in the duct. Must be installed parallel to the flow and either with the flow or against it.

Length of tube (m)

Sizing the pressure tubes (“measuring tubes”) for air and gases: Keep the tube as short as possible. An internal diameter of 4 mm is sufficient for pressure tubes of up to two metres in length. For longer pressure tubes, the internal diameter should be as indicated in the diagram. (Example: A pressure tube of 6 m requires an internal diameter of 6 mm.)

47

Pressure · Air

Connect the sensor and measuring instrument to the same point.

The tapping point must not be located where it will be affected by obstructions to the flow.

48

Pressure · Air

Where more than one sensor is used, the sensors should be installed on the same plane in the flow, and not in a position where one device will obstruct the air flow to the other.

dg = Equivalent diameter, page 45 Leave sufficient clearance downstream of any obstacles.

49

Pressure · Room

Pressure

Room The end of the pressure tube leading into the room should be protected by an air-permeable cover. Seal gaps between cable/plastic tubing and conduit. Otherwise measurements will be falsified by incorrect circulation of the air.

Pressure

Outside air

Pressure

Suction

Measure the outdoor pressure in an area sheltered from wind. Individual façades are not suitable measurement locations, as the pressure varies according to the wind direction. The correct location for measurement is a place where the air can circulate freely, such as a flat roof. Note, however, that the sensing point must be fitted with a wind shield. Options: Calculate average based on pressure measurements taken on several façades. Measure pressure in an open space (min 1.5 m above ground level). Multiple sensing point on flat roof.

50

Pressure tapping point: Sensing hole: diameter 5 mm, drilled and deburred.

Liquids

Pressure · Liquids

Pressure

Smooth interior (no burrs).

Use a damping coil to avoid transferring vibrations. Bend a 1 m long copper pipe, 4…6 mm in diameter, into a spiral with loops with a diameter of 15 cm.

Wrong: Air bubbles and condensate remain trapped.

51

Pressure · Liquids

Wrong: Condensate cannot be drained.

Installation in conjunction with liquids: Always install the pressure sensor in a location which is lower than the sensing point.

Installation in conjunction with vapours/gases Always install the pressure sensor in a location which is higher than the sensing point. 52

Pressure · Liquids

Pressure measurement in conjunction with liquids Do not measure at the top of the pipe (trapped air or air bubbles) or at the bottom (dirt).

The correct location for a sensing point is at the side.

Condensing gases Measure at the top to prevent condensate from entering the pressure tube.

53

Flow velocity · Air

Flow velocity

Formula:

Air The pressure-tapping point must not be located in turbulent air. Ensure sufficiently long settling-zones upstream and downstream of the tapping point. A settling-zone consists of a straight section of pipe or duct, with no obstructions.

dg = Equivalent diameter

Fan-belt monitoring The differential pressure across the fan is only suitable for fan-belt monitoring. ■ Negative connection (–) on suction side use copper tube ■ Positive connection (+) on discharge side: use Pitot tube Flow monitoring Flow detectors (electrothermal) Electrothermal flow detectors must be installed in a zone with a high flow velocity, e.g. where pipes narrow.

Differential pressure Do not monitor flow or differential pressure where flow resistance is variable, e.g. at filters, cooling coils, fans etc. Suitable locations: heating coils, silencers, baffles, attenuators.

54

Water Correct installation

Flow monitoring · Water

Flow monitoring

If differential pressure is used to monitor the flow, it is important to ensure that there are no stop valves or balancing valves between the sensing points. Incorrect installation

55

Air quality · Room

Air quality sensors

Room sensors There are two types of air quality sensors: Mixed gas (or VOC) sensors The accumulation of up to 24 different gases is measured in the ambient air (total concentration measured) Selective gas sensors These measure only one gas (e.g. CO2) in the ambient air (selective measurement). Do not locate temperature or humidity sensors above or below the AQ sensor.

The heated sensor element produces significant intrinsic heat in the device. Owing to this characteristic, the room temperature or room humidity must not be measured in the immediate vicinity. Maintain a clearance of minimum 60 mm on each side of the AQ sensor. CO2 sensors Selective gas sensors may require maintenance at regular intervals. Please consult the manufacturer’s instructions. The sensor must be installed in an accessible location.

56

Air quality · Room

Avoid recesses (e.g. shelving) and alcoves.

Do not install directly adjacent to doors.

Do not install behind curtains.

Seal gaps between cable/plastic tubing and conduit. Otherwise measurements will be falsified by incorrect circulation of the air.

57

Liquid level

Liquid level sensing A distinction is made between the following methods: ■ Capacitive measuring probes ■ Pressure/differential pressure (hydrostatic) ■ Ultrasound ■ Tank weighing systems ■ Electromechanical sensing

Measured variable

Pressure detector measures liquid level

Atmospheric pressure Compressed air

Measured variable

58

Pressure measurement: The pressure is defined by the height of the liquid medium, measured from the sensor to the surface of the liquid. There are no critical factors to consider when installing the pressure sensor. The sensor material must be suitable for use with the liquid medium. Bubble technique: The pressure measurement is determined by the height of the liquid above the bubble-tube outlet.

Liquid level

Floats: These are used in open and sealed systems. The device head must be installed above the maximum expected liquid level.

Tank weighing system The measured result is determined by the tank content (mass weight). The sensor should be mounted in accordance with the manufacturer's instructions.

Capacitive measuring probes: These are used in open and sealed tanks. Mounting: The distance from the next electrically conductive component must be as specified by the manufacturer.

Conductance systems: These are used in open and sealed tanks. The measured result is determined by the length of the immersed electrodes. Mounting: The distance from the next electrically conductive component must be as specified by the manufacturer. 59

Liquid level

Magnetic level switches: These are used in open and sealed tanks. Mounting: The liquid-level tube and the location of the level switch are determined by the manufacturer. Level switches are installed at the same height as the required liquid level.

Hydrostatic level sensing: The measurement is based on the maximum liquid level and the location of the sensor.

Location of sensor

60

General All measuring systems transmit physical variables such as temperature, humidity, pressure etc. with a particular response characteristic. The “response characteristic” (e.g. Tt = dead time / T = time constant or “lag”) refers to the reactions of the measuring systems.

Principles of operation

Principles of operation

Transfer with dead time, Tt, e.g. mixed-water temperature As the valve is adjusted by a given stroke, the temperature of the mixed water in the valve changes without any time delay. However, the valve and the temperature sensor are some distance apart. This is the distance the mixed water has to travel before the sensor can detect the change. This “transportation time” is referred to as “dead time”.

Tt = Dead time

61

Principles of operation

Time constant of measuring sensors in liquids For measuring or acquiring medium temperatures in piping systems, sensors are usually installed with protection pockets. These pockets represent the first delay element in the measuring process, the air gap between pocket and sensing element the second. The third delay element is the sensor’s time constant. Of these three delays in series, that of the air gap between pocket and sensing element is the greatest since the heat conductivity of air is poor. This poor transmission of heat from the medium to the sensing element can be considerably improved by filling the air gap with oil or glycerin. If glycerin is used, the welded protection pocket must be inclined.

Transfer with time constant = T

Change in measured variable

No sensors transmit the change in a measured variable instantaneously. The delay in transmission time (the time constant, or “lag”, T) can be shown in diagrammatic form.

Time constant T

The time taken to transmit 63% of the total change in the measured variable is referred to as the time constant, T. It takes a period equivalent to five times the time constant to transmit approximately 99% of the change in measured variable.

62

Sensor in water without immersion pocket

63 % 1,6 sec

Principles of operation

Example of response characteristic

Sensor in immersion pocket with contact fluid

63 % 16 sec Sensor in immersion pocket without contact fluid

63 %

Contact fluid

60 sec

Sensor in pipe Output signals The sensor converts the measured variable into an output signal. Sensors (measuring devices) are divided on the basis of their output signals into two main categories: Switching devices, examples: Thermostats, hygrostats, pressure switches Stepless (continuously variable) signals, examples: Temperature sensors, humidity sensors, pressure sensors

63

Principles of operation

Switching devices Switching devices On/off

Thermostats Hygrostats Pressure switches

Changeover

3-position

Multi-stage

Dynamic switching differential

Setpoint

Where a switching device such as a thermostat is installed in a system, it should be noted that the temperature swing will be wider than the switching differential of the thermostat. The thermostat operates at the switching points specified in the data sheet (static switching differential), but the inertia of the system (residual heat, dead time etc.) causes the controlled variable to overshoot or undershoot. The finally measurable temperature swing (the dynamic switching differential) will therefore always be wider than the static switching differential of the thermostat. 64

Stepless devices (sensors) produce a continuously variable, or “stepless” output signal. A given output value is assigned to each measured variable (temperature, humidity, pressure etc.) and the output signals are standardized. Normally, pressure, current, voltage or resistance signals are used for this purpose.

Principles of operation

Stepless devices

Pneumatic control system Pressure output signal: 0.2 ... 1.0 bar

Measured variable

Sensors used in electronic control systems operate with various output signals. Output signal Current : 0 ... 20 mA / 4 ... 20 mA

Measured variable

Output signal Voltage: 0 ... 10 V / 0 ... 1 V

Measured variable

Output signal Resistance: Various resistance values [ohms]

Measured variable

65

Temperature

Temperature Deflection of metals Metals respond to a change in temperature by a corresponding expansion (deflection). Sensors can be constructed in various ways to transfer this response. Bimetal strips: A bimetal strip is composed of two strips of metal with different coefficients of expansion, bonded together. As the temperature changes, one material (A) expands more than the other, causing the strip to bend. The curvature can be converted into an output signal.

Material

Construction

Bimetal rod Steel/brass Invar/brass

Metal A = Large expansion Metal B = Smaller expansion

66

Function

System – Deflection

Material

Construction

Tube Steel/brass Invar/brass

Function

System – Deflection

Temperature

Bimetal rod and tube

The tube (metal A) increases in length as the temperature rises. The Invar rod (metal B) does not change in length, with the result that the linear expansion of the tube is transmitted as a change in position. This, in turn, can be converted into an output signal.

Metal A Metal B (Invar)

67

Temperature

Force/deflection response of liquids and gases Liquids and gases also expand in response to a change in temperature. Various types of construction can be used to convert this expansion into a change in position. The output signal is derived from the change in position (potentiometer, inductive deflection, baffle plates etc.).

Material

68

Construction

Function

System

Liquid e.g. mercury, alcohol

Force/deflection

Gas e.g. helium

Force/deflection

Vapour For small measuring ranges

Force/deflection Non-linear due to to latent heat involved in a change change of state (phase transformation)

Liquid “averaging” type sensors The total length of the capillary is active.

Force/deflection

Material

Construction

Function

Platinum Nickel

Temperature

Electrical resistance sensors Metals change their resistance (measure in ohms) with a change in temperature. The change in resistance per Kelvin (K) is different for every metal, and is used directly as an output signal.

System Electrical resistance, R, in ohms Ω

There are two types of change in resistance: PTC: Positive temperature coefficient Rising temperature = increased resistance Falling temperature = reduced resistance NTC: Negative temperature coefficient Rising temperature = reduced resistance Falling temperature = increased resistance

69

Temperature

The relationship between the measured variable (temperature) and the resistance value is shown on a graph. The designation of the various PTC sensor elements is standardized and comprises: a) the material of the sensor element b) its resistance at 0 °C

e.g. Ni 1000 Nickel sensor element 1000 Ω resistance at 0 °C.

e.g. Pt 100 Platinum sensor element 100 Ω resistance at 0 °C.

NTC sensors The characteristic curve of the NTC sensor is not linear. The measuring ranges are defined by the manufacturer.

70

Construction

Function

Temperature

Thermocouples These consist of two wires of dissimilar metals, welded together at one end. System Voltage U in mV

Examples: Copper (Cu)/Constantan, Iron (Fe)/Constantan, Chromium (Cr)/Constantan, Nickel (Ni)/Constantan etc. A voltage (mV) is generated as a function of temperature at the welding point or “junction”.

71

Humidity

Measuring humidity Humidity is generally measured by use of hygroscopic materials. i.e. materials which respond to changes in humidity. Relative humidity (% r.h. = ␸) Principle: physical change in length Textile fibres (cotton, nylon etc.) expand as a function of relative humidity (producing a deflection).

% r.h. Cotton fibre

The deflection can be used as follows: To move a pointer (hygrometer), to operate a switch (hygrostat), to adjust a potentiometer, to adjust nozzles/baffles etc. (pneumatics), and to change inductivity (electrical variable).

Humid air

Insulating plate

Pole

72

Principle: change in electrical capacitance Capacitive elements respond to relative humidity by changing their electrical charge-storing capacity. Two electrical poles are connected to an insulating plate. In conjunction with the moist air, these poles store the electrical charge. The charge-storing capacity between the two poles depends on the ambient humidity.

Humidity

Absolute humidity x g H2O per kg of dry air

Salts (such as lithium chloride, LiCl) have hygroscopic properties. Their electrical conductivity changes according to the amount of moisture absorbed by the salt. A temperature sensor is wrapped with in a woven fabric which holds the salt. The fabric is wound with two non-touching electrical wires connected to an alternating current. As the air humidity rises, the moisture content in the salt increases, so reducing the electrical resistance between the wires. The smaller the resistance, the higher the electrical current, so that the two wires act as an electric heating element. The heat produced causes the water to evaporate. As the moisture content is reduced, so the resistance increases again, and the heat output is reduced. This alternating process is repeated until the amount of water evaporated is equal to the amount of water absorbed. At this point, a state of equilibrium (constant temperature) is reached. This temperature (the transformation temperature) is a measure of absolute humidity. Combined humidity sensors There are also combined humidity sensors on the market, which operate simultaneously with the temperature and humidity measurement principles described above. By measuring the temperature and relative humidity, these sensors can be used to calculate the dew point (°Tp) or the moisture content (g H2O). Alternatively, the relative humidity (␸) can be calculated on the basis of the temperature and the moisture content (g H2O) or dew point (°Tp). 73

Pressure

Measuring pressure Force/deflection systems With pressure sensors of this type the pressure is converted into a change in position or force. This can be used to: ■ Move a pointer ■ Operate a switch ■ Adjust a potentiometer, etc.

There are various ways of converting the pressure into a deflection. The main methods are: ■ Bellows: Pressure switches or manostats for higher pressures ■ Bourdon tube: Manometers ■ Diaphragm: Fine-pressure measuring instruments ■ Aneroid barometer: Fine-pressure measuring instruments

Bellows

74

Bourdon tube

Diaphragm

Barometer

Pressure

Electronic systems Piezoelectric elements: The force acting on the quartz crystals generates an electrical charge. This charge is converted into an output signal corresponding to the pressure. Thermodynamic pressure sensors: The amount of air flowing through the pressure tube varies according to the differential pressure. The air is heated by a heating element with a constant heat output. The higher the differential pressure, the more air will flow through the tube, thereby reducing the heating effect. The change in temperature is measured by two sensor elements, and provides a measure of the differential pressure.

Output Quartz

Quartz

Heating element

Important: Take care to size the differential pressure tube correctly (see page 47).

75

Velocity

Measuring air velocity and air flow (Measured variable w = m/s) Velocity measurements based on effective differential pressure A fixed obstacle (orifice plate, flow nozzle etc.) placed in the air flow creates a pressure differential which varies according to flow velocity. The flow velocity can be determined by measuring this differential pressure and converting it appropriately. Special flow detectors are available for this purpose:

Orifice plate

Flow nozzle

Flow cross

Flow grid

Hot-wire anemometer (thermoelectric) A heating element is heated to a given temperature. The heat output required to maintain this temperature varies according to the flow velocity. The required heat output is measured and converted into a flow velocity. Important: The hot-wire anemometer is only suitable for spot measurements. 76

Like air velocity, water velocity can be measured with orifice plates. Velocity measurements, however, are primarily required in order to determine heat volume. There are various commercially available heat meters, based on different mechanisms for measuring flow velocity.

Velocity

Measuring water velocity and flow

Example:

Orifice

Impeller

Ultrasound

Magnetic flux Inductiv

Important: The required steadying zones must be allowed for, upstream and downstream of the sensor. Flow velocity To determine the flow velocity, the differential pressure at the flow detector (orifice plate, flow grid etc.) is measured. The same general mounting instructions apply as for differential pressure sensors. Important: The required steadying zones must be allowed for, upstream and downstream of the sensor.

Formula: dg = Equivalent diameter

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Selecting the right sensor

Selecting the right sensor Temperature measurements in pipework Immersion sensors: In most cases, the immersion sensor offers the greatest advantages. The entire active length of the sensor must be fully immersed in the pipe. Strap-on sensors Used in cases where installation conditions make the immersion sensor unsuitable. For retrofit installation (renovation projects). For heating systems in residential buildings. Cable sensors: The cable sensor has significant advantages (e.g. sealing) in refrigerant pipes. Temperature measurement in tanks Important: Avoid measurements at outer extremities of tank (20 cm). Capillary sensors with These sensors are particularly suitable where probes/cable sensors a clearance is required between the sensor and the device head due to vibration, space problems or temperature conditions. Probe sensors: Generally suitable for this application. Temperature measurement in ducts Probe sensors: These produce spot measurements and should be used only in ducts where there is no stratification. Averaging sensors:

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Recommended for all applications. The length of the rod or capillary tube must be suitable for the duct cross-section (approximately 4 m per m2). Never install sensors with plastic-coated sensor elements downstream of electric heating coils. Where this is unavoidable, fit a radiant-heat shield.

Selecting the right sensor

Room temperature measurement Room sensors: In large spaces, it is advisable to use more than one sensor (for an average value). With high internal loads and where extractors are used above lamps, sensors must not be sited in the exhaust air flow. Important: Remember to take account of reheating in the duct Air quality sensors: Do not install near heat sources. Outdoor temperature measurement Outdoor sensors: Suitable for heating systems. In ventilation systems, the temperature must be measured directly after the weather shield in the outside air intake. Measuring humidity The time constant (lag coefficient, or response time) of the humidity sensor can vary from 10 s…5 mins, depending on the type of sensor. As a general rule, sensors with a short response time (less than 1 minute) should be used for measurements in the supply air. Table of lag coefficients for various sensor elements: Sensor element: Approx. lag coefficient Man-made fibres 1…3 min Cotton 3 mins Lithium chloride 110 s Capacitance 10…20 s Fast humidification systems: supply air, steam humidifiers, spray humidifiers. Important: Solvents in laboratories, chlorine in swimming pools, disinfectants in hospitals etc. will impair the service life and operation of humidity sensors. Air velocity: The maximum admissible air velocity at the sensing point of a humidity sensor must not be exceeded.

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Selecting the right sensor

Measuring pressure and differential pressure of gases (air) and liquids The nominal pressure PN of the pressure sensor must correspond to the safety pressure of the system. The maximum permissible load on one side must not be exceeded. The pressure sensor must be approved for use with the medium to be measured (water, vapour, refrigerant, foods, gases etc.). The measuring range must be selected so that the set value is not at the start or end of the scale. Measuring velocity and volumetric flow rate Various measuring systems are available for measuring velocity or volumetric flow rate. The key factor here is whether a spot measurement or an average is required. Measuring systems: Spot measurements: Pressure tube Hot-wire anemometer Vane anemometer

Suitable for: Gases, liquids Gases Gases, liquids

Averaged measurements: Orifice plates Flow nozzles (cross) Flow nozzles (ring) Ultrasound Magnetic flux Helical flow Ring piston

Suitable for: Gases, liquids Gases Gases Liquids Liquids Liquids Liquids

With spot measurements, the measured result is closely dependent on the flow profile. This is why it is strongly advisable when measuring velocity and volumetric flow rates, to use measuring systems of the “averaging” type. 80

The process is always based on a comparison. In order to calibrate sensors, a high-quality measuring instrument must be used. This type of check is only useful if the measured variable remains constant throughout the calibration process. Important: Avoid external influences (e.g. heat gain from the tester's own body etc.) The calibration process must be carried out directly on the sensor. The verification of sensors in conjunction with the electrical installation must be carried out only by qualified personnel (see regulations).

Calibration

Calibration of measuring sensors

Calibrating a frost thermostat The loop on the capillary tube (20 cm) is immersed in a vessel filled with water and ice cubes. This “iced water” is measured with the thermometer. Set the frost protection thermostat to the measured temperature. The frost protection thermostat should now trip at this temperature (recalibrate if necessary). Now set the frost protection thermostat to +2 °C above the frost protection setpoint.

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Calibration

Clinical syringe

∅p filter

Calibrating a filter monitor Use a clinical syringe to check the filter monitor as follows: Method: ■ Switch off the system. ■ Disconnect the differential pressure tubes (+ and –) from the pressure-tapping points. Connect the syringe and a manometer (U-tube) to the “+” side. ■ Switch on the system. ■ Gradually increase the pressure with the syringe until the manometer shows that the switching point has been reached. ■ The alarm device should trip at this point. (Recalibrate if necessary.) ■ Switch off the system. ■ Re-connect the differential pressure tubes. ■ Switch on the plant again. 82

Practical tips

Practical tips Frost protection thermostats Purpose: Frost protection thermostats are designed to prevent water-filled heat exchangers from freezing. Method: The air-side frost temperature must be monitored with a capillary sensor.

Construction

Function

System Force/deflection

The capillary frost-protection sensor operates on the principle of vapour pressure condensation. If the temperature falls below the preset switching point of 2 °C at any point along any 10 cm length, the vapour pressure in the sensor system drops, causing the frost alarm to trip.

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Practical tips

Basic circuit: If the air temperature downstream of the heat exchanger falls below the preset temperature, then: ■ the supply/extract fans switch off ■ the outside air/exhaust air dampers close ■ the heating coil valve opens ■ the internal pump switches on ■ the frost alarm trips. The frost alarm is self-locking, to prevent the system from being switched on again automatically by the frost thermostat. The alarm must be acknowledged locally (remote acknowledgement is prohibited). To prevent overheating inside the unit, the heating coil valve is regulated by the frost protection thermostat while the frost alarm is active. Tips for avoiding nuisance frost alarms: The main cause of a frost alarm is a drop in temperature. It is possible, however, for a frost thermostat to trip even when the heat supply is adequate. This may be due to one of the following circumstances: a) Changes in load, such as a change from one fan-stage to another, or a heat recovery system coming on/off load b) Temperature stratification c) System start-up after a shutdown To prevent the frost thermostat from tripping in situations where the temperature is adequate, the following basic circuits have proven effective: ■ Preventive frost protection thermostat ■ Stabilised start function ■ Setpoint-regulated start-up control

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Practical tips

Preventive frost protection thermostat: The preventive frost protection thermostat operates in two phases. Open

Temperature Frost alarm

Closed Valve

Example: 1. If the air temperature around the frost thermostat drops below 6 °C, the thermostat takes over control of the heat exchanger valve to maintain the air temperature at a value between 2 °C and 6 °C. 2. If the air temperature is still below the 2 °C switch-off point, (e.g. due to insufficient heating), the frost alarm will trip.

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Practical tips

The stabilised start function and setpoint-regulated start-up control are used if there are additional heat exchangers (heat stores) installed downstream of the preheater. Stabilised start function: The stabilised start function maintains the air downstream of the heating coil at a minimum temperature. To achieve this, an averaging sensor must be installed directly adjacent to the frost-thermostat capillary. The minimum-temperature controller can be set independently of the frost thermostat and acts directly on the preheater valve.

A: Selector function

Supply air 2

Setpoint-regulated start-up control: Setpoint-regulated start-up control requires an averaging sensor, installed directly adjacent to the frost-thermostat capillary. If its temperature falls below the value selected on the out-of-limits sensor, the latter adjusts the setpoint used for control of the supply air.

Break point for start-up sensor

Setpoint Supply air 1

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Practical tips

Pressure control in VAV systems The sensing point must be sited at the most remote point in the duct system. In duct systems with a large number of branches, the use of several sensing points is recommended (the lowest pressure is used for control). If some parts of the ductwork (zones) are shut off by dampers, the relevant sensors must be disconnected. The setpoint can only be maintained at the sensing point.

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Practical tips

Average measurements in water pipes Average measurements are recommended in all pipes where stratification occurs (e.g. downstream of a mixing point), or where the sensor needs to be installed as close as possible to the mixing point (to reduce lag). For averaging purposes, either four sensors can be distributed round the circumference of the pipe or an averaging sensor can be wrapped round the pipe.

These arrangements should be used in systems with large pipe diameters and variable volumes of water.

Electrical connection of 4 averaging sensors for measurement of the average temperature.

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